Successful drilling, completion and production of an earthen wellbore requires that information be gathered about the downhole formation from which hydrocarbons are produced. Measurement systems are lowered into a drilled wellbore to determine wellbore parameters and operating conditions. A portion of the measurement system includes a sensor package for detecting the wellbore parameters and conditions, such as formation properties, tool and borehole direction, drilling fluid properties, dynamic drilling conditions, and others. The sensor package may be lowered on a tool body after the drill string is tripped out of the borehole, such as with a typical wireline operation. Alternatively, the sensors may be housed in a drill collar and adapted for taking measurements while drilling, as in certain applications known as measurement-while-drilling (MWD) or logging-while-drilling (LWD). In addition to the sensor portion, a sensor tool may also include a processor and associated storage medium for retaining the sensed information. With respect to a MWD/LWD tool, a telemetry system is often used to transmit the sensed information uphole. The telemetry system may include a mud pulser, an acoustic telemetry option, or an electromagnetic transmission system.
The sensors and associated electronic and mechanical components are packaged within the tool body. For example, the sensors and detectors may be hardwired within the tool body and accessible via removable hatches. In another arrangement, the sensors are mounted upon a chassis and retained within an outer housing or sleeve. Such arrangements place certain tool components between the interior sensing and logging devices, and the target formation or fluids exterior of the tool. Sensitive logging devices, such as nuclear measurement devices using gamma rays, can be affected by the intervening tool components. Furthermore, the intervening hatches, housings, sleeves, drill collar material, stabilizer sleeves and the like place the nuclear sources and sensors further from the formation.
In addition, the varying demands of the hydrocarbon field require expensive measurement tools to be useable across different tool bodies and drill collars, including tool bodies and drill collars having different sizes. Therefore, it becomes necessary to deploy a nuclear measurement tool that addresses these shortcomings as well as others in the field.